In the conventional "helical-scan" field of writing and reading television signals on magnetic tape, the tape is moved longitudinally in a helical path around a rotating drum that carries a pair of magnetic heads, which scan tracks that cross the tape diagonally at an acute angle to the tape length, the angle being determined as a compound function (vector resultant) of the longitudinally-directed tape speed and the diagonally-directed head speed. If, during the read mode, the tape is slowed down to provide a "slow-motion" effect, or speeded up to provide a "fast-motion" effect, or stopped to provide a "stop-motion" or "still frame" effect, the head still scans the tape diagonally-transversely, but at a slightly offset angle to the tape tracks. To cause the head to follow a previously-recorded track under such circumstances, it has been usual to mount the head on the end of an arm that deflects under magnetic or electric influence, or that bends transversely to the plane of the drum, and to derive a signal from the continuously reading head to indicate its lateral position with respect to the track being read. That is, a signal indicating that the head is either dead-on the track, or off-center. This signal then is used to control the deflection of the arm to keep the head centered on the track. Aside from controlling head tracking during fast, slow or stop-motion reading modes, this system is also useful in correcting for inadvertent wandering of the head from the track as a result of differences in recording-reproducing machines or operational variations in the tape speed, or as the result of shrinking or extension of the tape length due to temperature or humidity changes between the times and places of recording and reproducing.
Such a structure is taught in U.S. patent application Ser. No. 179,161 filed Apr. 8, 1988, by the present assignee. This structure was particularly devised to overcome limitations of the typical bimorph leaf arrangement, namely a restricted range of lateral motion and a limited speed of response to deflection corrections, which makes the bimorph less suitable for use in apparatus requiring very-high-speed head deflections. For such high-speed deflections, it was desired to use a metal leaf operated by electro-magnetic driving means. But some experimental leaves proved to have undesired resonance vibrations. To solve this problem, the invention of Ser. No. 179,161 now abandoned made the leaf member extremely stiff throughout the greater part of its length, and flexible only in a limited zone adjacent to its bending-pivoting anchorage at the base member to which it was attached. This arrangement provided a substantial increase in the range or width of the band of operating frequencies, and improved the "jump speed" of the automatic-scan-tracking apparatus. That is, it improved the ability of the head to leave the end of a track at one edge of the tape and to jump back to the correct lateral position for approaching the other tape edge, and also for picking up the beginning of the same track or the next, depending on whether the tape is stopped or is moving in normal, slow or fast motion mode. It will be understood that in, for example, a 180-degree helical-scan apparatus, the track itself is wrapped in a helical path around slightly more than 180 degrees of the drum circumference, and two heads are used, each of which which scans the tape for 180 degrees of its circuit and then moves free of the tape for the next 180 degrees while the other head is scanning. During the time of free movement, the head on its arm must make the so-called lateral "jump" to correct for any mis-alignment caused by the stopping, slowing or speeding-up of the tape, or by its having shrunk or stretched.
However, it has been the object of continuing research to obtain the advantages of a pantographically connected pair of parallel leaves, as an alternative to the single leaf of Ser. No. 179,161.
The primary benefit of an ideal pantographic structure is to maintain the head perpendicular to the tape at all deflections.
Another advantage of the pantographic structure is that a head deflection sensing means can be mounted on the head-carrying portion of the pantograph, and at a convenient shorter radius, while maintaining the ratio of head-deflection to sensing-means deflection at a value of unity, so as to achieve an optimum signal-to-noise ratio. Examples of structures subject to or solving this problem include U.S. Pat. Nos. 4,337,492, 4,099,211 and 4,212,043.
One problem, however, that sometimes is encountered with existing pantographs, is that the parallel arms are made to be stiff and rigid (see U.S. Pat. No. 4,212,043), and bending stresses are concentrated at four points--two for each of two pantograph arms.
In some uses, such concentration of stresses may be tolerable. However, in the use intended for the present invention, such concentration of repeated bending stresses would engender the danger of early failure, for the arms must be made extremely thin in order to increase the resonant frequencies of the arms to values above the zone in which they might cause instability in the operation of the position servo mechanism. With such thin arms, the repeated flexings at concentrated points would contribute to much shorter fatigue life.
With a pair of un-stiffened, flexible arms, however, each arm can be flexed in an S-shaped curve, and the system may be operated at high speed, yet the bending stresses are distributed along the length of the arm, and the arm life is maintained at a much higher value.
With two elongated flexible arms, however, the ensemble, in the exacting uses contemplated for the present invention, may be extremely sensitive to torsional deflection and vibration of the arms about their longitudinal axes, which can throw off the tracking of the head both directly and also indirectly, as by affecting the operation of the head-position sensing means that is needed to control and correct the tracking (see for example, the arrangement of U.S. Pat. No. 4,363,046).
Not only dynamic torsional deflection has been a problem in the prior art, but also permanent torsional deformation, such as often occurs when the apparatus is being cleaned by hand. If the structure cannot resist such deformation, it may become "sprung" beyond its elastic limit, like the hinge of a door, and can be bent back into its originally manufactured conformation only with great difficulty. It often may better be replaced entirely, for clearly, to operate the system in "sprung" condition would be to set the head at an incorrect azimuth with respect to the tape track, to produce relative displacements between the head gaps in a multiple-gap head stack, and more importantly, to interfere with smooth track-to-track operation of the apparatus.
A final problem with some prior art pantographic designs is that, when subjected to the high-speed flexures at which the present invention is meant to operate, the two arms might have nearly the same harmonic resonating frequencies, and when such a frequency is reached, the action of each arm might tend to exacerbate the action of the other, and a "beating" effect may occur that is much more severe than it would be for either arm alone.
Accordingly, the present invention advantageously improves the accuracy of orientation of the head with respect to the tape, improves the frequency response and jump speed of a mount which moves a magnetic transducing head transversely across a magnetic tape track, achieves a servo gain ratio of unity, improves the resistance of such a mount to torsional deformation, and avoids the tendency for beating resonance between the two arms of a pantographic mount.